Re: [PATCH v2] tracing: Fix race condition in kprobe initialization causing NULL pointer dereference

On Mon, 29 Sep 2025 12:12:59 +0200
Peter Zijlstra [off-list ref] wrote:

On Mon, Sep 29, 2025 at 11:38:08AM +0206, John Ogness wrote:

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Problem:
1. CPU0 executes (1) assigning tp_event->perf_events = list

smp_wmb()

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2. CPU0 executes (2) enabling kprobe functionality via class->reg()
3. CPU1 triggers and reaches kprobe_dispatcher
4. CPU1 checks TP_FLAG_PROFILE - condition passes (step 2 completed)

smp_rmb()

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5. CPU1 calls kprobe_perf_func() and crashes at (3) because
   call->perf_events is still NULL

The issue: Assignment in step 1 may not be visible to CPU1 due to
missing memory barriers before step 2 sets TP_FLAG_PROFILE flag.

A better explanation of the issue would be: CPU1 sees that kprobe
functionality is enabled but does not see that perf_events has been
assigned.

Add pairing read and write memory barriers to guarantee that if CPU1
sees that kprobe functionality is enabled, it must also see that
perf_events has been assigned.

Note that this could also be done more efficiently using a store_release
when setting the flag (in step 2) and a load_acquire when loading the
flag (in step 4).

The RELEASE+ACQUIRE is a better pattern for these cases. 

And I'll argue the barrier should be in 2 not 1, since it is 2 that sets
the flag checked in 4.  Any store before that flag might be affected,
not just the ->perf_events list.

RELEASE+ACQUIRE ensures the memory ordering on the `same` CPU, so do we still need smp_rmb() and smp_wmb()? e.g.

perf_trace_event_init()
-----
tp_event->perf_events = list
/*(1)*/
smp_store_release(&tp->event->flags, newflag);
-----

kprobe_dispatcher()
-----
smp_load_acquire(&tk->tp.flags);
/*(2)*/
this_cpu_ptr(call->perf_events);
-----

This ensures
 - the flags update is shown on other CPUs
 - perf_events update is done before the flags update on the same CPU
 - perf_events load is done after the flags load on the same CPU

But we are still not sure the perf_events update is shown on other CPUs?

To ensure that we still need smp_wmb() and smp_rmb() at (1) and (2).

Or we don't need smp_*mb()?

Thank you,

-- 
Masami Hiramatsu (Google) [off-list ref]